HST/STIS observations of the optical counterpart to GRB970228

We report on observations of the fading optical counterpart of the gamma-ray burst GRB970228, made on 4 September 1997 using the STIS CCD on the Hubble Space Telescope. The unresolved counterpart is detected at V=28.0±0.25, consistent with a continued power-law decline with exponent 1.14±0.05. No proper motion is detected, in contradiction of some earlier claims. The counterpart is located within, but near the edge of, a faint extended source with diameter ∼0.8 and integrated magnitude V=25.7±0.25. Comparison with WFPC2 data taken one month after the initial burst and NTT data taken on March 13 shows no evidence for fading of the extended emission. After adjusting for the probable Galactic extinction in the direction of GRB970228 of Av∼0.7, we find that the observed nebula is consistent with the sizes of galaxies of comparable magnitude found in the Hubble Deep Field and other deep HST images, and that only 2% of the sky is covered by galaxies of similar or greater surface brightness. We therefore argue ...